The present invention relates to a speed control system for an engine and, more specifically, relates to a speed control system which protects accessory equipment from damage resulting from operation at excessive speeds by disconnecting accessory equipment from the engine upon sensing an overspeed condition.
There are many situations where special purpose accessories are driven by gasoline or diesel engines capable of speeds exceeding safe speed of operation of the accessory. For example, a truck engine may be used to drive an accessory unit, such as an hydraulic pump. Typically, such truck engines are capable of speeds far in excess of speeds falling within the design limits of the accessory unit. Accordingly, the accessory unit may be damaged when the driving engine exceeds the maximum design speed of the accessory. Additionally, a problem is presented upon engagement of the accessory to the driving engine in these and other instances, since the accessory may be damaged not only by operating the accessory or the interface above a safe speed, but also by engaging it above a safe engagement speed.
Electronic speed switches are commercially available today and these provide some measure of overspeed control. However, they also usually require a sending unit such as a tachometer, mechanically linked to the engine, in order to provide overspeed sensing. Another deficiency in known speed control switches is that they typically permit the accessory to be reconnected to the driving engine at a speed below the maximum safe operating speed, but above a safe speed for engagement of the accessory to the engine. As indicated above, an accessory may be damaged not only by operating it at an excessive speed, but also by attempting to connect or disconnect it at speeds above idle but below the maximum safe accessory operating speed. Therefore, a need exists for some means to prevent accessory damage caused either by operating over speed or engaging at an unsafe speed. Further, extended operation at about the maximum safe speed can produce rapid, repeated engagement and disengagement of the accessory resulting in failure of the accessory and/or the interface in a relatively short time.